[eng] The main objective of this thesis is to analyse the variation in non-breeding social systems within the group of parids and to identify the main factors involved. More specifically, I studied the role of food hoarding on the structure of winter social systems at different scales. The study was focused on the social patterns and hoarding of two species of tits inhabiting boreal (the Willow tit, "Parus montanus") and montane subalpine forests (the Coal tit, "Parus ater"). The results included in this thesis are mainly concerned with the Coal tit, because the Willow tit has been studied in some detail by other researchers in the same study area described below. Therefore, most of the points here investigated for the Coal tit were already studied for the Willow tit.